Fixing my S4

Ok, I'll try to make this short and sweet. So my S2 finally died. I searched high and low and decided to buy an S4 from a guy on Craigslist. It was a Galaxy S4 locked to AT&T. When I met him I checked out the device briefly. Seemed to work fine. Paid him $200. He claimed he just wanted the money to buy the new iPhone. Ok, so fast forward. I had to buy a new activation kit from Family Mobile as it is and has been my carrier and needed to be converted to a micro SIM. Also tried calling both my carrier and AT&T to see if they would unlock it. Then paid $30 for Unlockcell.net I think it's called to unlock the phone. It did say I remember that the phone was successfully unlocked still could not receive or make calls, although the whole time the phone I could use for internet. I took it to 2 different cell phone repair places, who told me the IMEI number had been tampered with, the sticker identifying the phone had been removed, and the phone was likely been stolen. Well, I'm $260 in at this point. My question is if I get the model number and replace the motherboard, will this phone now be a working phone with my carrier? My mom bought me the new S5 for Christmas, although I bought a Concord TMoblile phone after this whole debaucle. I would like to make this phone functional and resell it but don't want to buy or invest any more money into it if I have the same result. I'm also not about scamming people on Craigslist. I now have 5 phones sitting around the house and just want to recoup or get this one working where someone else can use it. Please any expert or advice would be helpful. THANKS SO MUCH FOR ANY INPUT!

Welcome to Android Central! If the phone's IMEI has been tampered with, then it's likely been blacklisted anyway, and you won't be able to use it as a phone, period. Since there's a good chance it was stolen, probably the best course of action is to report it to the police, and see what they want you to do with it. Sorry!